1. Field of the Invention
This invention pertains to the generation of electrical energy by magnetohydrodynamics, employing hot ionized gases as the working fluid, and more particularly to an electrode structure suitable for use in such a device.
2. Summary of the Prior Art
Mucn of magnetohydrodynamics (hereafter, and more conventionally, MHD) for converting thermal energy into electrical envisages the use of hot gases as the conductive fluid medium. From the pure MHD viewpoint, liquids are much simpler to handle; highly conductive liquids are common, and any that are corrosive can usually be withstood by some available conductive electrode material, as evidenced by numerous MHD pumps. But liquids do not remain liquid over a thermodynamically desirable temperature range. Efficient conversion of thermal energy into electrical energy by MHD compels the use of hot gases which are usually oxidizing, possibly otherwise corrosive from fuel impurities, and are ordinarily seeded with reactive materials such as cesium to promote ionization; they are ordinarily above the melting points of available metals. Some ceramics, such as certain spinels, are not only resistant to this inhospitable atmosphere, but have the fortunate property of being electrically conductive at high temperatures. Other ceramics have electrical conductivity sufficiently low even at high temperatures to make them satisfactory insulators. Also, refractory members are required to form gas passages. All three have the common limitation that their exposed faces must be cooled to keep them below their melting or deformation temperature, which is not so high as the gas temperature to which they are exposed. This may be done by cooling the reverse of the ceramic by attaching it with low thermal resistance (and, for the electrodes, with low electrical resistance) to a metal cooling block. This may be done by brazine the ceramic to the metal cooling block. However, several problems arise. The ceramic itself will tend to arch outward toward the hot side because of differential expansion, and also to pull away from the cooling block; and because even the cooled reverse will not match the cooling block in its thermal expansion, it will also tend to break the brazed bond by sliding parallel to the interface. The ceramist has negligible liberty to adjust this formulation to produce a desired thermal expansion, because he must conform to cogent requirements for thermal and prescribed electrical conductivity.
The text Open-Cycle MHD Power Generation, editors J. B. Heywood and G. J. Womack, Pergamon Press, 1969, L. C. Catalogue Card No. 73-79462, in section 7.3.5, pp. 554 through 558, describes electrodes of plasma-sprayed zirconia applied to the ends of a brush of nickel alloy wires in one case, and to a mesh of patinum wire in another case. Both of these electrodes were cooled at the reverse by a flow of oxygen. The brush electrode had cracks in the zirconia which permitted alkali seed material to penetrate the device; and oxidation of the nickel alloy brush itself by the cooling oxygen was severe. The mesh electrode performed somewhat better, but the face of the zirconia was severely eroded by electrochemical action.